Typical, commercially available, glass fiber thermal and acoustical insulation comprises solid, single-glass, straight, and relatively short glass fibers, made by a rotary fiberizing process. A single molten glass composition is forced through the plurality of orifices in the spinner peripheral wall and primary fibers are produced. These are further attenuated by an annular main burner and compressed-air blower combined actions. A binder is sprayed onto the fibers as they are drawn downwards. The fibers are collected on a foraminous conveyor with bottom suction and then, as wool pack, fed into the curing oven for thermal processing (drying and binder thermal setting).
Light density, small diameter glass fibers about 5 microns average diameter are properly assembled into a three-dimensional fibrous lattice or structure, commonly referred to as a wool pack or web.
Light density glass fiber insulation product is highly compressible and resilient. The air content of a light density, 0.7-pcf, glass fibre insulation product approaches 96% vol. (0.7-pcf product, glass density=160 pcf (2,560 kg/m3), glass volume percent=(0.7/160)*100%=4.4%, air content (volumetric percent)=95.6%.
Spatial fibrous lattice is effective in trapping air between fibers, and prevents any air flow. The convective mode of heat transfer is therefore virtually eliminated. Heat can still be transferred through stagnant air by conduction but it is not an effective way of heat transport. Thermal conductivity of air is only 0.023 W/(m*K), and for light density glass fiber insulation it is the thermal resistance of still (stagnant) air trapped inside the three-dimensional fibrous structure (matrix) that determines the ability of insulation product to retard heat transfer. As the product density goes up, the solid conduction through the glass fibrous lattice starts to become a factor, since glass thermal conductivity is 0.800 W/(m*K), i.e. 30-times more than that of still air.
The fine fiber lattice retards heat transfer also by scattering radiation. The more uniform fiber spatial distribution and the higher the fiber surface area (finer fiber), the more intense radiation scattering, and the better thermal insulating capability.
Insulating materials generally rely on entrapped still air for the majority of their insulating qualities; the thermal conductivities for commonly used insulating materials typically lie in the range of 0.018 to 0.046 W/(m*K) for insulating product in a dry state. If water or water vapor enters the insulant, its thermal conductivity will increase significantly. Water has a very detrimental effect on insulating material ability to resist heat flow. The thermal conductivity of water is 0.580 W/(m*K). The K-value of light density fibrous insulation materials can increase by 35% with only 1.5% moisture entering into the material.
The open structure of fibrous insulating materials provides little resistance to water or water vapor penetration. Closed cell insulants are, certainly, less sensitive to water penetration; however plastic (polyethylene) film vapor barriers are used when fiberglass insulation is installed in residential homes.
The apparent thermal conductivity of a typical insulating material somewhat exceeds the thermal conductivity of the gaseous medium itself, i.e. the gaseous medium which is made stagnant inside the insulating material body. Thermal conductivity of still gas is considered to be the lower limit. Wood (dry) thermal conductivity is λ=0.400 W/(m*K), magnesia MgO insulation=0.070, cork=0.070, saw dust=0.060, paper=0.050, wool=0.050, foam glass=0.045, rockwool=0.045, glass wool (fiberglass)=0.040, kapok insulation=0.034, expanded polystyrene=0.030, cotton=0.030 and still air=0.026. Thermal conductivity λ of water is 0.600 W(m*K), for glass, the λ numerical value is 0.800 W/(m*K). Thermal conductivity of carbon steel is 54 W/(m*K).
Since the apparent thermal conductivity of insulating material depends on the thermal conductivity of the gas, the obvious approach to lower the overall thermal conductivity of insulation material is to reduce the thermal conductivity of the gas, either by lowering the gas pressure (vacuum), or using gases with thermal conductivities lower than that of air. Some gases of potential interest include (the relative thermal conductivities, with respect to air, given in brackets, air=1.000): argon Ar (0.677), ethane C2H6 (0.750), propane C3H8 (0.615), carbon dioxide CO2 (0.620), hexafluorosulfide SF6 (0.500). Some of these gases (hydrocarbons) are flammable, so their consideration is rather doubtful. This approach is used in practice to make closed cell insulations (foamed plastics and foam glass).
As previously mentioned, air has very low thermal conductivity, and some other gases, such as carbon dioxide and Freon, have even lower thermal conductivities. The problem with gases is that they are subject to convection currents, and they are transparent to infrared radiation. Gases need to be contained within a structure to prevent convection taking place and to block heat transmission by infrared radiation, if the advantage of low thermal conductivity of “still” gas is to be achieved.
Until the early 1900's, it was thought that the only way to provide a thermal insulation with a lower thermal conductivity than still gas (air) would involve the use of a vacuum. Around 1920 a theory was developed for a method of making an insulation material which could have a thermal conductivity lower than still gas. The governing idea was to prevent the collision between hot and cold gas molecules by creating some extremely thin solid barriers, separated by a distance smaller than a gas molecule mean free path. In the case of air at 0° C. the mean free path is 60*10−9 m.
This imagined structure became reality in 1940's when Samuel Kittler from Monsanto, produced a silica aerogel. The silica chains provided the barriers needed to give the microporous structure for an insulation material with a thermal conductivity lower than that of still air. The silica was ground to a powder and sold as a pourable insulation under the Monsanto trade-names Santocel A™ and Santocel C™ from the 1940's to about 1970.
Lower cost silicas, known as pyrogenix or fumed silicas, were later produced by burning silicon tetrachloride in the presence of hydrogen. In the 1950's, the first handleable microporous block-type insulation was made by Johns-Manville for nuclear and aerospace applications. The product was called Min-K™, and comprised Santocel™ silica. These were reinforced with asbestos fibre, and bonded with organic resin. Since then some further improvements were made in the microporous insulation manufacturing process, eliminating the use of asbestos and organic resin. An opacifying powder was uniformly distributed throughout the silica structure to reflect, refract or absorb infra-red radiation. Thermally stable metal oxide opacifyers of particle size comparable to the wavelength of incident radiation, scatter (back-scatter) the infra-red radiation, and therefore still further reduce the radiative heat transfer.
Silica aerogel density can be as low as 3 kg/m3. Water density equals 1,000 kg/m3. Granular aerogels have only a small point of contact between granules in an aerogel bed, and thus the result is an extremely low solid conductivity component. The radiative mode of energy transport can be suppressed by adding a second component to either absorb or scatter (reflect) the incident infra-red (thermal) radiation. Elemental carbon (carbon black) is an effective infrared radiation absorber.
A typical silica aerogel has a total thermal conductivity of 0.017 W(m*K), i.e., R10/inch; for vacuum, aerogel (50 Torr) thermal conductivity can be lowered to 0.008 W/(m*K), R20/inch. At ambient pressure, a 9% wt. carbon black addition lowers the thermal conductivity from 0.017 to 0.014 W/(m*K). If gas pressure is reduced to 50 Torr, the aerogel thermal conductivity can be as low as 0.004 W/(m*K).